If you add the age of a man to the age of his wife you get a combined age of 91 years. He is now twice as old as she was when he was as old as she is now.
What is the difference between their ages?
Explanations
He is 52. She is 39. Back when he was as she is now, i.e. 39, she was 26, i.e. twice less than he is now.